Thermoelectric materials can convert an appreciable amount of thermal energy into electricity in an applied temperature gradient (e.g., the Seebeck effect) or pump heat in an applied electric field (e.g., the Peltier effect), in the solid state and with no moving parts. The applications for solid-state heat engines are numerous, including the generation of electricity from various heat sources whether primary or waste, as well as the cooling of spaces or objects such as microchips and sensors. Interest in the use of thermoelectric materials that comprise thermoelectric devices has grown in recent years in part due to advances in nano-structured materials with enhanced thermoelectric performance (e.g., efficiency, power density, or “thermoelectric figure of merit” ZT, where ZT is equal to S2σ/k and S is the Seebeck coefficient, σ the electrical conductivity, and k the thermal conductivity of the thermoelectric material) and also due to the heightened need for systems that either convert waste heat to electricity to improve energy efficiency or cool integrated circuits to improve their performance.
To date, thermoelectrics have had limited commercial applicability due to the relatively poor cost performance of these devices compared to other technologies that accomplish similar means of energy generation or refrigeration. Where other technologies usually are not as suitable as thermoelectrics for use in lightweight and low footprint applications, thermoelectrics often have nonetheless been limited by their prohibitively high costs. The manufacturability of thermoelectric devices and modules is important in realizing the usefulness of thermoelectrics in commercial applications. These modules are preferably produced in such a way that ensures, for example, maximum performance at minimum cost.
The thermoelectric materials in presently available commercial thermoelectric modules generally include, or are comprised of, bismuth telluride or lead telluride, which are both toxic, difficult to manufacture, and expensive to procure and process. With a strong present need for both alternative energy production and microscale cooling capabilities, the driving force for highly manufacturable, low cost, high performance thermoelectrics is growing. However, many drawbacks may exist in the production of conventional thermoelectric devices.
Nanostructures have shown promise for improving thermoelectric performance. Nanostructures often refer to structures that have at least one structural dimension measured on the nanoscale (e.g., between 0.1 nm and 1000 nm). For example, a nanowire is characterized as having a cross-sectional diameter that is measured on the nanoscale, even though the nanowire may be considerably longer in length. The creation of 0D, 1D, or 2D nanostructures from a thermoelectric material may improve the thermoelectric power generation or cooling efficiency of that material in some instances, and sometimes very significantly (a factor of 100 or greater) in other instances. However, many limitations exist in terms of formation and handling of the nanostructured materials needed for making an actual macroscopic thermoelectric device. The ability to process nanostructures associated with a common semiconductor material like silicon would have tremendous cost advantages for making large scale application possible for thermoelectrics.